Fiber lasers or fiber amplifiers contain an optical fiber as laser-active medium, the fiber core of which is doped with a laser-active material such as neodymium Nd, ytterbium Yb, or erbium Er. This doped fiber core is enclosed by so-called pump light cladding into which the pump light is injected. The advantages of such a fiber laser or fiber amplifier lie in its simple design, high excitation efficiency, efficient cooling due to its large surface area, and high beam quality which can be attained.
High beam quality is attainable in particular when a monomodal or fund amental mode fiber is used as optical fiber, the laser-active fiber core of which has a diameter in the range of only several μm. For such a small diameter of the fiber core, only the fundamental mode TEM00 which has optimal beam quality can oscillate in such a small diameter of the fiber core.
However, a disadvantage of the small diameter of the fiber core required herefor is that, at such a small diameter, high laser power is associated with correspondingly high power densities. These power densities may result in undesired nonlinear effects, and may even lead to destruction of the optical fiber. To allow higher power to be achieved, it is therefore necessary to enlarge the fiber core. However, this is accompanied by a degradation of the beam quality. At fiber core diameters greater than approximately 20 μm, oscillation of higher modes may be suppressed only with great difficulty.